This invention relates to electrochemical cells having metal anodes and air cathodes, commonly known as metal air cells. More specifically, this invention relates to air cathodes and the current collecting substrates thereof, and methods of hardening such substrates prior to their introduction into metal air cells.
The recent increase in small electrically powered devices has increased the demand for very small electrochemical cells, usually disc-like or pellet-like in appearance, and commonly referred to as button cells. Such cells, which are often approximately the size of garment buttons, have diameters ranging up to about 1.0 inch and heights ranging up to about 0.60 inches. The small size and limited amount of electrochemically-active material contained in these small metal-air cells results in considerable attention being directed to improving the capacity of such cells.
Metal air cells are electrochemical cells where the oxygen in the air is the cathode material and a metal material is the anode material. In many instances the preferred anode material is zinc. Metal air cells convert atmospheric oxygen to hydroxide in the air cathode, the hydroxide then migrating to the anode, where it causes the metal contained therein to oxidize. In metal air cells, air enters the cell through one or more air ingress holes in the bottom of the cathode container, the holes generally being in close proximity to an air cathode disposed within the cell. Air diffuses into the air cathode, where the oxygen in the air reacts with water to form hydroxide.
Generally, the air cathode of a metal air cell is composed of a mixture of catalytically active materials disposed upon a current collecting substrate capable of being connected to electrical circuitry, most often through a conductive cathode container. More particularly, the catalytically active materials typically are composed of manganese dioxide and carbon, and the current collecting substrate usually comprises a cross-bonded screen having nickel strands woven therein, or a fine mesh expanded nickel screen.
Several prior art disclosures have been made suggesting the use of nickel screens in air cathodes for electrochemical cells, or suggesting shotblasting or sandblasting of wire, including:
______________________________________ Country Patent Number Inventor/Applicant Issue Date ______________________________________ U.S.A. 3,746,580 Aker et al. 1973 U.S.A. 4,209,574 Ruetschi 1980 U.S.A. 4,254,593 Paulfeuerborn 1981 U.S.A 4,343,869 Oltman et al. 1982 U.S.A. 4,369,569 Dopp 1983 U.S.A. Re. 31,413 Jaggard 1983 ______________________________________
Aker et al., in U.S. Pat. No. 3,746,580, disclose a gas depolarizable galvanic cell having a metallic grid or screen, preferably of nickel, onto which is pressed a porous, wet or liquid proofed catalyst composition comprising carbon and a wet polymer.
In U.S. Pat. No. 4,209,574, Ruetschi discloses a primary alkaline cell having a nickel screen disc onto which is disposed silver oxide or mercuric oxide.
Paulfeuerborn, in U.S. Pat. No. 4,254,593, discloses a machine for shot or sand blasting wire or rod-like materials.
Oltman et al., in U.S. Pat. No. 4,343,869, disclose an improved metal oxygen button cell having a mesh, film, or screen current collector comprising conductive metal such as nickel or stainless steel.
In U.S. Pat. No. 4,369,568, Dopp discloses an improved metal air button cell having a mesh, film, or screen current collector comprising a conductive metal such as nickel or stainless steel.
In U.S. Pat. No. Re. 31,413, Jaggard discloses a button type gas depolarized electrochemical cell having a current collecting member or screen.
We discovered that the internal resistance of metal air cells is a function, inter alia, of the amount of physical and electrical contact between the catalytically active materials and the current collecting substrate. As the amount of such contact decreases, cell internal resistance increases. One means of increasing the amount of physical and electrical contact between the catalytically active materials and the current collecting substrate is to increase the amount of surface area provided by the current collecting substrate, and available for the attachment of catalytically active materials thereto.
The pulse capability of metal air cells is also a function of the amount of electrical and physical contact between the carbon and the current collecting substrate. The greater the amount of such contact, the lower the electrical resistance therebetween, and the greater the pulse capability. High cell internal resistance may also cause a voltage deficiency at high rates of current drain therefrom, or when large short-lived current pulses are withdrawn therefrom.
We tested many different commercially available metal-air cells and found that their internal resistances were often too high for applications requiring high current drain rates. In those tests, typical IR losses across the internal resistances of the cells averaged around 100 mV when a 150 ohm (8 mA) load was connected thereacross. Because the closed circuit voltage of a cell having such a large internal resistance averages around 1.1 volts under a 150 ohm load, and because many devices like hearing aids have cut-off voltages between about 1.05 and 1.10 volts, a small increase in cell internal resistance can render the cell incapable of producing voltage sufficient to operate such devices.
High internal resistances in button cells often result from structural deformation of the current collecting substrate during crimping of the cell upon closure, where the deformation is of sufficient magnitude to cause inadequate physical and electrical contact between the substrate and the interior sidewall of the cathode container. Additionally, when a current collecting substrate deforms structurally during crimping of a cell upon closure, the outer periphery of the sealing gasket attached to the bottom surface of the air cathode often deforms in accordance therewith, thus increasing the likelihood of electrolyte leaking from a cell.
In metal air cells, the cathode is catalytic and not consumable. Hence, additional capacity for metal air cells is typically provided for by adding more metal anode material and electrolyte. In metal air cells, therefore, capacity is usually limited by the amount of anode material in the cell. That is, cell capacity is directly related to the amount of electrochemically active and consumable anode material present in the cell, the amount depending in turn on the internal cell volume available for the anode material. An important objective of metal air cell developers, therefore, is to decrease the volume occupied by the various internal cell structural components, thereby increasing the amount of internal cell volume available for anode material.
One volume-consuming internal cell structural component is the current collecting substrate. Previous attempts to reduce the volume of this structure often led to a decrease in cell performance, however, because they were usually directed solely to decreasing the diameter of the metal wire comprising the current collecting substrate. Using such decreased diameter wire often caused a marked increase in impedance failures because substantial deformation of the wire typically occurred upon crimping of the cell during closure.
Therefore, it is an object of the present invention to increase the amount of physical and electrical contact between the catalytically active material and the current collecting substrate of air cathodes in metal air cells.
It is another object of the present invention to increase the amount of physical and electrical contact between the current collecting substrates and the inner sidewalls of cathode containers in metal air cells.
It is yet another object of the present invention to increase the surface areas of current collecting substrates in metal air cells.
It is still yet another object of the present invention to improve the performance of metal air cells under pulse and high current drain conditions.
It is yet still another object of the present invention to decrease the number of metal air cells made having high internal resistance due to structural deformation of the current collecting substrate.
A further object of the present invention is to increase the hardness of current collecting substrates in metal air cells, thereby increasing their strength and rigidity.
A further object yet of the present invention is to reduce the number of metal-air cells made having electrolyte leaking therefrom by increasing the strength or rigidity of the current collecting substrate.
Yet a further object of the present invention is to reduce the volume of the current collecting substrate, thereby increasing the internal capacity of metal-air cells so made.
Other objects and advantages will become apparent from the following summary and description of the invention.